1. Field of the Invention
The present invention relates generally to a hydraulic power boost apparatus for use with an automotive vehicle, and more specifically to a hydraulic power boost apparatus having a means for limiting the travel of the slide valve within the apparatus to prevent damage to the resilient power cushion positioned within the apparatus.
2. Description of the Prior Art
Various hydraulic power boost mechanisms are known to generally comprise a booster housing in substantially cylindrical form, a power piston sealingly and slidably disposed within the booster housing and a spool valve disposed within the power piston and connected to the brake pedal. The power piston is formed with a plurality of annular recesses in the internal bore thereof. The spool valve is also formed with a plurality of annular recesses on the outer periphery thereof. Each of the power piston internal bore recesses cooperates with the spool valve outer recesses to selectively regulate the flow of hydraulic fluid therethrough. In such a hydraulic power boost mechanism, by applying the brake pedal, pressure fluid flows into the booster housing through an inlet port and into a pressure chamber formed therein. The accumulated fluid pressure shifts the power piston to actuate a power transmitting rod connected to a tandem brake master cylinder. When the brake pedal is applied to a predetermined force, the spool valve engages the bottom of the power piston so that force applied to the brake pedal is directly transmitted to the power piston. At this time, the spool valve interrupts communication between the inlet port and the outlet port.
In the conventional hydraulic power boost mechanism, if the brake pedal is applied with a force in excess of the predetermined force, the spool valve first closes communication between the inlet port and the pressure chamber and thereafter engages the bottom of the outlet port and establish communication between the inlet port and the power piston. Between the point of closing the spool valve and the bottoming out point, there is a loss stroke. Specifically, when the spool valve reaches the position where the first valve just completely closes, a clearance remains between the top of the spool valve and the bottom of the power piston bore. Thereafter, when the boost pressure is fully applied to the power piston, the spool valve still moves slightly within the piston bore until it reaches the bottom. The foot force is then directly transmitted to the brake tandem master cylinder with a corresponding reaction force applied to the brake pedal. This increases the braking "back force" directed to the driver's foot.
For preventing the above-mentioned disadvantages, there have been proposed improved hydraulic power boost mechanisms having a resilient member within the bottom of the power piston. The thickness of the resilient member corresponds to a dimention of loss stroke of the spool valve during which the spool valve travles to the bottom of the piston under no reactance force. In such a construction, when the spool valve just closes the communication between the inlet port and outlet port, the top of the spool valve engages the resilient member to thereby shift the power piston. Until the resilient member is fully deformed, the force applied to the master cylinder and the reaction force to the brake pedal are gradually and moderately increased.
In this construction however, the resilient member can prevent the brake system from rapidly increasing the braking force. Also, the resilient member is apt to be damaged by repeated deformation or by being deformed beyond its resilient limit, since the size of the resilient member is limited by the relatively narrow space within the power piston. This will decrease the durability of the boost mechanism and require frequent replacement of the resilient member.